<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    #myInput {
    background-image: url('https://static.runoob.com/images/mix/searchicon.png'); /* 搜索按钮 */
    background-position: 10px 12px; /* 定位搜索按钮 */
    background-repeat: no-repeat; /* 不重复图片*/
    width: 100%; 
    font-size: 16px; /* 加大字体 */
    padding: 12px 20px 12px 40px; 
    border: 1px solid #ddd; 
    margin-bottom: 12px; 
}
 
#myUL {
    /* 移除默认的列表样式 */
    list-style-type: none;
    padding: 0;
    margin: 0;
}
 
#myUL li a {
    border: 1px solid #ddd; /* 链接添加边框 */
    margin-top: -1px; 
    background-color: #f6f6f6; 
    padding: 12px; 
    text-decoration: none;
    font-size: 18px; 
    color: black; 
    display: block; 
}
 
#myUL li a.header {
    background-color: #e2e2e2; 
    cursor: default; 
}
 
#myUL li a:hover:not(.header) {
    background-color: #eee;
}
    </style>
</head>
<body>
    <input type="text" id="myInput" onkeyup="myFunction()" placeholder="搜索...">

<ul id="myUL">
  <li><a href="#" class="header">A</a></li>
  <li><a href="#">Adele</a></li>
  <li><a href="#">Agnes</a></li>

  <li><a href="#" class="header">B</a></li>
  <li><a href="#">Billy</a></li>
  <li><a href="#">Bob</a></li>

  <li><a href="#" class="header">C</a></li>
  <li><a href="#">Calvin</a></li>
  <li><a href="#">Christina</a></li>
  <li><a href="#">Cindy</a></li>
</ul>
</body>
<script>
    function myFunction() {
    // 声明变量
    var input, filter, ul, li, a, i;
    input = document.getElementById('myInput');
    filter = input.value.toUpperCase();
    ul = document.getElementById("myUL");
    li = ul.getElementsByTagName('li');
 
    // 循环所有列表，查找匹配项
    for (i = 0; i < li.length; i++) {
        a = li[i].getElementsByTagName("a")[0];
        if (a.innerHTML.toUpperCase().indexOf(filter) > -1) {
            li[i].style.display = "";
        } else {
            li[i].style.display = "none";
        }
    }
}
</script>
</html>